Vascular development consists of two phases: vasculogenic creation of vessels by endothelial progenitor cells and subsequent angiogenic remodeling of existing vessels. Mice lacking the hematopoietic signaling protein Slp-76 develop a vascular phenotype in which nascent lymphatic vessels connect to preexisting blood vessels. Our studies of the cell type responsible for this vascular phenotype suggest that Slp-76 is required in hematopoietic endothelial progenitor cells and not in mature endothelial cells. Remarkably, blood-lymphatic connections are corrected during later lymphatic remodeling, leaving surviving animals with arterio-venous shunts in which mesenteric lymphatics carry blood. These congenital lymphatics subsequently lose lymphatic identity, suggesting that fluid flow forces are sufficient for the molecular reprogramming of lymphatic vessel identity in vivo. These observations lead us to hypothesize that an early vasculogenic phase of lymphatic growth utilizes hematopoietic endothelial progenitors while lymphatic vessels can be remodeled and even alter their vessel identity in response to blood flow during later angiogenic phases of vessel growth. In the present proposal we will test these hypotheses and address the nature of the molecular defect by which loss of Slp-76 confers vascular mixing. Our specific aims include: Aim 1. To determine the contribution of hematopoietic cells to developing lymphatic endothelium using genetic lineage tracing. Aim 2. To test the role of Slp-76 in the maturation and function of endothelial progenitors. Aim 3. To determine if fluid shear forces alter lymphatic endothelial identity. These studies will shed light on the basic mechanisms of lymphatic vascular development, the use of hematopoietic cells as endothelial progenitors, and the role of blood flow in establishing vessel identity.This proposal will investigate a new pathway by which blood cells contribute to the formation of lymphatic vessels. Lymphatic vessels are needed to remove fluid from tissues and edema secondary to lymphatic vascular insufficiency is a common side effect of radiation treatment and infectious diseases. These studies are therefore relevant to the development of new ways to treat of lymphatic vascular diseases.